Papillary Immature Metaplasia of the Anal Canal: A Low-grade Lesion That Can Mimic a High-grade Lesion.

In a natural history study of anal human papillomavirus (HPV) infection and HPV-related lesions among homosexual men in Sydney, Australia, we identified 15 examples of papillary immature metaplasia (PIM) in anal biopsy samples. PIM has previously been described in the cervix, but not in the anal canal. PIM is a form of exophytic low-grade squamous intraepithelial lesion (eLSIL) also known as condyloma. In contrast to the maturing keratinocytes and koilocytosis seen in conventional eLSIL, the slender papillary structures of PIM have a surface population of immature squamous cells. In our anal samples PIM was characterized by close proximity to conventional eLSIL, was negative for p16 (p16) expression, and revealed the presence of a single low-risk HPV genotype (either 6 or 11) in laser capture microdissected lesions. The clinical significance of recognizing PIM lies in preventing misdiagnosis as high-grade squamous intraepithelial lesion, (the presumed precursor to anal cancer), due to the morphologic immaturity of the cell population. In routine practice, awareness of anal canal PIM and p16 immunostaining will prevent this. Further study of the natural history of anal canal PIM is needed.